Heat Transfer in Nucleate Pool Boiling under Microgravity

用乙醇进行了微重力条件下核池沸腾实验。介绍了一种带有传感器的透明加热表面，用于测量生长气泡下的局部表面温度和液膜厚度。表面采用蓝宝石玻璃，透明ITO膜加热器背面加热结构，实现了对附着气泡底部液-气行为的观察，同时精确评估了局部表面热流密度。提出了一种测量液膜厚度的新方法，即用一组薄电极直接涂覆在受热面上。采用4mm方形、0.1 μ m厚度的传感器，可瞬间测量1 ~ 10 μ m量级的微层厚度。测量在压力0.01至0.02 MPa，液体过冷0至27 K，热流密度高达90000 W/m^2的条件下进行。澄清如下。i)在低热通量区微重力条件下，随微层是否出现干斑，存在着换热增强和变差两种相反的趋势。ii)在离散气泡区域，大气泡下方的局部换热系数受重力水平的影响显著，而整个受热面的平均系数则不敏感。iii)在高热流密度时，在微重力条件下，在一个大的聚并气泡下面，小的单个气泡从受热面出现，并在大层表面塌陷。换热主要以宏观层内的核沸腾为主，无论重力水平如何变化，换热基本保持不变。结果解释了近30年来已有的实验结果，即微重力下的平均换热系数随实验方式和实验条件的不同而增大或减小，并假设其值与地球上的值相差不大。

Experiments in nucleate pool boiling under microgravity were conducted by using ethanol. The transparent heating surface with sensors for the measurement of local surface temperature and liquid film thickness underneath a growing bubble was introduced. The surface is made of sapphire glass, and the structure of back-side heating by transparent ITO film heater realizes the observation of liquid-vapor behaviors at the base of attached bubble and simultaneously the accurate evaluation of local surface heat flux. A new method was developed for the measurement of liquid film thickness by using a set of thin electrodes coated directly on the heating surface. The sensor of 4mm square and 0.1 mum thickness was introduced to measure instantaneously the microlayr thickness of 1 to 10 mum order. Measurements were performed under pressure 0.01 to 0.02 MPa, liquid subcooling 0 to 27 K,heat flux up to 90000 W/m^2.The following are clarified. i) In microgravity at low heat flux region, there exists two opposite trends of heat transfer enhancement and deterioration depending on whether dry patches emerge or not in the microlayr. ii) In the discrete bubble region, the local heat transfer coefficients underneath a large bubble are significantly influenced by the gravity level, while the coefficient averaged over the entire heating surface is quite insensitive. iii) At high heat flux, underneath a large coalesced bubble in microgravity, small single bubbles emerge from the heating surface and collapse at the surface of macrolayr. The heat transfer is dominated mainly by the nucleate boiling in the macrolayr, and it remains almost constant despite the change of gravity level.The results give the interpretation of existing experimental results in the past three decades, in which the average heat transfer coefficients in microgravity increase or decrease depending on the experimental manner and conditions adopted, and assumes the values not far different from the terrestrial ones.